Devitrification Dikes and Giant Spherulites Froni Klondyke, Arizona1
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,N MINERALOGIST, VOL. 47, ]ULY-AUGUST, 1962 DEVITRIFICATION DIKES AND GIANT SPHERULITES FRONI KLONDYKE, ARIZONA1 Fner.Ir S. SrnroNs,U. S. GeologicalSuraey, Federol Center, Denaer,Colorad,o, Aesrxlcr Vitrophyric rhyolite welded tufi near Klondyke, Arizona, contains thin dike-like sheets and spheroids as much as 6 feet across that have formed by devitrification of the original glassy rock. Chemical analyses show the devitrified tufi to have markedly more potash and less soda than its vitreous equivalent. Devitrification is believed to have been an autometamorphic process brought about by passage of potassium-rich fluids along fractures during weiding and cooling of the tuff. A similar chemical trend is noted in some devitrified rocks of Great Britain. fNrnonucrtor.t During the course of mapping the Klondyke quadrangle, Arizona, some striking devitrification features were noted in a rhyolite welded tuff. A cursory review of the literature reveals only scattered observa- tions of similar features, and it therefore seemsworthwhile to call atten- tion to the exceptionally well-exposedexamples at Klondyke. GBor,ocrc SnrrrNc The Klondyke quadrangle is in Graham County, Lrizona, about 55 miles northeast of Tucson. The northeast quarter of the quadrangle is occupied by parts of the north-northwesterly-trendingSanta Teresa- Turnbull Mountains mass,the bulk of which is made up of sedimentary rocks of Paleozoicand Late Cretaceousage, plulonic igneousrocks of Precambrian and Tertiary age, and metamorphic rocks of Precambrian age. Along the west flank of the mountainsis a thick sequenceof domi- nantly silicic volcanic rocks of possibleLate Cretaceousto early Tertiary age which either are faulted against or rest unconformably on the older rocks. The rock in which the devitrification features were noted is ex- posedalong the east side of a gully in the center of sec. 18, T.6 S., R. 20 E., about 2,000feet eastof the road to Imperial lVlountainand 5 miles north of Klondyke. In this area the volcanic rocks are silicic lavas, flow breccias, tufis, and welded and partly welded tuffs that strike N. 25"-30"W. and dip 55'-60'SW. One unit of the sequenceis a lens of black vitrophyre as much as 40 feet thick that is exposedfor 900 to 1,000feet along the strike (Fig. 1). The lens appears to pinch out to the northwest and is covered by al- luvium at its southeast end. The vitrophyre is a hard glassy rock with subconchoidal to somewhat hackly fracture and contains scattered 1 Publication authorized by the Director, U. S. Geological Survey. 87r 872 FRANK S. SIMOTS Frc. 1. Lens of vitrophyric rhyolite welded tuff (B) dipping 55o-60o to the right and somewhat toward the observer. The lens is about 40 feet thick. It is underlain by pale red partly welded tufi (A) and overlain by grayish red welded tufi (C). The devitrification "dikes" are best exposed on the steep slope below the highest outcrop; the giant spherulites crop out along the entire slope and seem to be concentrated near the top of the vitrophyric welded tuff just beneath unit C. Arrows point to outcrops of several giant spherulites. millimeter-sized phenocrysts of feldspar and sparse biotite and quartz It gradesdownward through 15 20 feet of mottled black and red partly vitrophyric welded tuff to a sharp contact with pale redl partly welded rhyolite tufi 10 30 feet thick. The vitrophyre lens is overlain by grayish red solidly welded and almost completely devitrified rhyolite tuff of unknown but appreciable thickness. All these rocks are believed to be parts of a single tuff deposit, the lithologic variations being due mainly to differencesin degree of welding and devitrification. Under the microscope the vitrophyre consists of densely welded pale brown glass shards enclosing euhedral to subhedral grains of oligoclase, sanidine, and minor brown biotite and quartz, as well as sparsepumice lapilli and lithic fragments (Fig. 2). DBvrrnrrrcarroN DrKES The vitrophyric welded tufi is cut by numerous steep-dipping dike- like bodies of moderate brown devitrified rock that contrast strongly with the black vitrophyre (Fig. 3) and stand out in slight relief from the vitrophyre upon weathering. These "dikes" are confined to the vitro- phyre, although at least one "dike" structure is representedin the under- I This and succeeding colors are from the Rock Color Chafi, Nati.onal ResearchCouncitr, 1948. DEVITRIFICATION DIKES 873 Frc. 2. Photomicrograph of vitrophyric rhyolite welded tufi. {-plagioclase; b-biotite; l-lithic fragment; p-pumice; rest of field is pale-brown glass shards. The vitroclastic texture is clearly preserved but the shards are densely welded. Plane polarized light' X50. Frc. 3. Light colored devitrification "dikes" in black vitrophyric welded tufi. Scale is 7] inches long, widest "dike" is about 14 inches wide. AII these "dikes" pinch out uphill just above field of photograph. The vitrophyre between the two largest "dikes" is cut by several smaller "dikes" not clearly visible in the photograph 874 FRANK S. S/MOTS Frc. 4. Light-colored narrow devitrifica- Frc. 5. Photomicrograph of irregular tion "dikes" in black vitrophyric welded contact between vitrophyre (v) and "dike" tufi. Scale is 7] inches long. Note the thin of devitrified rock (d). Textures on both white median layer of quartz (q) in both sides of the contact are essentially identical "dikes," particularly the right-hand "dike." except where the original texture has been Left-hand "dike" continues uphill for 15-20 destroyed by growth of incomplete spheru- feet above field of photograph; both "dikes" lites, as along the upper right-hand segment pinch out just below field. of the contact. Plane polarized light, X33. lying partly welded tufi by a simple fracture. The "dikes" are rather ir- regular in width, ranging from a fraction of an inch to about 3 feet across;most are an inch or less wide. Near the southeastend of the vitrophyre lens, numerous ramifying "dikes" divide the vitrophyre into a mosaic of irregular blocks. All the "dikes" are divided symmetrically along their centers by a thin layer of qlrartz. The thickness of the quartz layer appears to be roughly proportional to the width of the enclosing "dike" but is less than 2 millimeters regardlessof "dike" width. Contacts between vitrophyre and "dikes" may be irregular with many sharp bends,re-entrants, etc., as shown in Fig. 3, or may instead be es- sentially straight for appreciable distances, as in Fig. 4. The contacts invariably are sharp, and no transition from stony to vitrophyric is de- tectable with a hand lens. Within the "dikes" themselves,however, the rock flanking the quartz core is slightly darker than that in contact DEVITRIFICATION DIKES 875 with the vitrophyre, the color changing gradually outward from mod- erate brown to light brown. The core rock also is more resistant to weatheringthan is that along the contacts,which on weatheredsurfaces commonly are marked by a shallowtrench resultingfrom the disintegra- tion of rock along both sidesof the contact. On a microscopicscale the contacts between,,dike,' and vitrophyre are exceedingly irregular, with narrow tongues of one rock penetrating the other (Fig. 5). The contactsare, however,quite sharp, and provide almost no evidenceof any transition betweenthe two rocks.Locally, the original vitroclastic texture has been obliterated by the formation of incomplete spherulites, but for the most part is clearly preserved.in the devitrified rock. only a few tiny islands of glassy rock have survived devitrification. Within the devitrified rock, none of the phenocrystic minerals appear to have been altered at all during devitrification. The quartz seamsalong the centersof the ,,dikes',consist of a core of granular qutrtz flanked by narrow irregular selvagesmade up of tiny rhomb-shapedgrains of adularia (?) in a fine-grainedmatrix that may be cristobaliteor tridymite (Fig. 6); in generalthe mineralsof the selvages are too fine grained to be identified positively in thin section. The straight sharply definedwalls of the quartz seamsand the completelack of any evidencesuggesting replacement of the host rock indicate that the seams must be true fissure fillings. The sequenceof development appears to have been: 1) fracturing of the welded tuff to produce narrow open fissures;2) depositionof a little very fine-grainedadularia (?) and cristobalite (?) or tridymite (?) along the walls of the fissuresland 3) complete filling of the fissures by quartz. Some of the quaftz was de- positedearly in the third stage,as witnessthe two grainswith hexagonal outlines near the left edge of Fig. 6, which must have been deposited prior to the final filling of the fissure. The minerals formed during devitrification are too fine grained to be identified by optical methods. A sample of devitrified tufi carefully hand picked to remove phenocrystic minerals was examined with the r-ray spectrometerby Theodore Botinelly of the U. S. GeologicalSurvey, who reports plagioclase feldspar, qvartz, cristobalite, orthoclase and mica. Chemical analyses of a devitrification ,,dike,, and of its immediately adjacent vitrophyric wall rock are given in Table 1, columns I and,2; these samples must represent essentially identical original rock. The principal chemical change accompanying devitrification obviously has been addition of potash and removal of soda; the ratio K2O:Na2O is about 1.4 in the glassyrock and 7.7 inthedevitrified, rock. The devitrified equivalent also is slightly more silicic and contains about 2 per cent less 876 FRANK S. 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